ABSTRACT
An antigen is a foreign substance capable of reacting with an antibody. A foreign substance capable of eliciting antibody formation when introduced into a host is called an immunogen. Natural immunogens are usually macromolecules such as proteins and polysaccharides. Other molecular structures can also act as immunogens, for example, glycolipids (such as A, B, and O blood group antigens) and glycoproteins (such as Rh and Lewis antigens). However, they must be foreign to their hosts. Another class of immunogens is called haptens. Although haptens are too small to elicit antibody production, a hapten-conjugated carrier can elicit the formation of an antibody specific for the hapten. Certain controlled substances such as cocaine and amphetamines are haptens and can be detected through corresponding antibodies for forensic toxicological analysis.
